Single-ended-to-differential converter with common-mode voltage control

ABSTRACT

Provided is a circuit to perform single-ended to differential conversion while providing common-mode voltage control. The circuit includes a converter to convert a single-ended signal to a differential signal and a stabilizing circuit adapted to receive the differential signal. The stabilizing circuit includes a sensor configured to sense a common-mode voltage level of the differential signal and a comparator having an output port coupled to the converter. The comparator is configured to compare the differential signal common-mode voltage level with a reference signal common-mode voltage level and produce an adjusting signal based upon the comparison. The adjusting signal is applied to the converter via the output port and is operative to adjust a subsequent common-mode voltage level of the differential signal.

[0001] This application is a continuation of the U.S. Non-ProvisionalApplication entitled “Single-Ended-To-Differential Converter withCommon-Mode Voltage Control,” Ser. No. 10/425,736, filed Apr. 30, 2003,which is a continuation of U.S. Non-Provisional Application entitled“Single-Ended-To-Differential Converter with Common-Mode VoltageControl,” Ser. No. 10/105,253, filed Mar. 26, 2002, all of which isincorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to single-ended to differentialconverters. More particularly, the present invention relates to acircuit technique to perform attenuation and single-ended todifferential conversion with common-mode voltage control on a singleintegrated circuit (IC).

[0004] 2. Background Art

[0005] Advances in electronic device packaging provide electronic systemdesigners with the ability to include an increased number of functionson a single IC. Such ICs are particularly well suited for low supplyvoltage applications, especially those ICs that can accommodateprocessing of analog signals. Thus, by combining numerous functions on asingle IC, electronic system designers are able to realize tremendoussavings in power consumption and space. Even greater functionaladvantages can be realized through the selection of particular signalprocessing techniques to be used on these ICs.

[0006] For example, in the case of analog signals on single ICs,differential signal processing is preferred over single-ended processingbecause it provides better rejection of power supply and substratenoises. Differential signal processing is also inherently better atrejecting common-mode voltages than single-ended processing. Forpurposes of illustration, differential signal processing is commonlyused in single-chip video receivers, also known as on-chip receivers. Inon-chip receivers, however, certain applications often benefit more fromthe input of single-ended signals instead of differential signals. Also,from the standpoint of hardware, single-ended signals may be preferredas circuit inputs because they are easier to provide as circuit inputsand can be more cheaply produced than their differential-ended versions.When single-ended signals are provided as circuit inputs, conventionalon-chip receivers normally include additional on-chip circuitry toconvert the received single-ended signals into differential signals inorder to facilitate the more advantageous differential signalprocessing.

[0007] A number of traditional techniques exist to convert single-endedsignals to differential signals. One popular technique includesconverting the single-ended signal from the voltage domain to thecurrent domain. With this particular technique, the single-ended signalis received at one of the inputs of a voltage to current converter. Inresponse, the converter produces at its output, a differential signal,having a positive signal component and a negative signal component. Bothsignal components, however, are in the current domain. To convert thisdifferential current signal from current domain to voltage domain, thedifferential signal is forwarded to a current to voltage converter, suchas resistor, in order to finally create the differential voltage signal.One deficiency with this approach is that it's not well suited for lowvoltage power supplies. More specifically, it's difficult to fit thisapproach within the voltage head-room constraints of low voltage powersupplies.

[0008] Another technique for converting single-ended signals todifferential signals includes the use of a degenerate differential pair.However, when providing one side of a degenerate differential pair witha relatively large signal, while at the same time, leaving the otherside at a constant voltage, its extremely difficult to achieveacceptable linearity. Providing one side of the degenerate differentialpair with a large signal and leaving the other side at a constantvoltage is required in order to convert a single-ended signal to adifferential signal.

[0009] In addition to single-ended to differential converters,attenuators are also circuits that are commonly used in on-chipreceivers. When used to attenuate input signals, the dynamic range ofattenuators can be adjusted to fit within the dynamic range ofsubsequent signal processing blocks, such as track and hold circuits oranalog-to-digital converters (ADCs).

[0010] Although numerous traditional techniques exist to convertsingle-ended signals into differential signals, low voltage powersupplies often do not provide adequate voltage head-room to accommodateefficient use of these traditional techniques. As a result, there is aneed for a device that can provide and improved approach for convertinga single-ended signal to a differential signal without the deficienciesof the traditional approaches discussed above. Also, there is a need toprovide an improved single-ended to differential conversion techniqueand an attenuator on a single IC chip to save power and optimize circuitboard space.

BRIEF SUMMARY OF THE INVENTION

[0011] Consistent with the principles of the present invention asembodied and broadly described herein, an exemplary circuit includes anattenuator having first and second receiving ports configured torespectively receive first and second input signals and differentialoutput ports configured to output a differential signal produced inaccordance with the first and second input signals. The attenuator alsoincludes an adjusting node positioned in association with thedifferential output ports. A stabilizing circuit, including a sensor anda comparator, is configured to receive the differential signal. Thesensor is coupled across the differential output ports and has afeedback node. The comparator includes two input ports and a comparatoroutput port. A first of the two input ports is coupled to the feedbacknode and the other of the two input ports is configured to receive areference voltage signal. The comparator output port is coupled to theadjusting node. The sensor is configured to detect a common-mode voltagelevel of the differential signal and provide the detected common-modevoltage level to the first input port. The comparator is configured tocompare the common-mode voltage level to a level of the referencevoltage signal and produce an adjusting voltage signal based upon thecomparison. Finally, the adjusting voltage signal is applied to theadjusting node and is operative to adjust a subsequent common-modevoltage level of the differential signal.

[0012] Features and advantages of the present invention include theability to beneficially combine the functions of a low powersingle-ended to differential converter and an attenuator on a single IC.This approach increases the performance and provides additional space onthe IC for incorporation of added functions. Additional advantages ofthe present invention are the ability to provide single-ended todifferential conversion in a manner that fits within the voltagehead-room constraints of low voltage power supplies and a provision forcontrolling common-mode voltage in an efficient manner.

BRIEF DESCRIPTION OF THE FIGURES

[0013] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate an embodiment of theinvention and, together with the description, explain the purpose,advantages, and principles of the invention. In the drawings:

[0014]FIG. 1 is a block diagram of an exemplary implementation of thepresent invention;

[0015]FIG. 2 is a schematic diagram of an exemplary circuit constructedand arranged in accordance with the present invention;

[0016]FIG. 3 is a schematic diagram of the circuit of FIG. 2 includingvoltage buffers;

[0017]FIG. 4 is a schematic diagram of a conventional amplifier used inthe circuits of FIG. 2 and FIG. 3;

[0018]FIG. 5 is a block diagram of a programmable gain attenuator usedin a preferred embodiment of the present invention; and

[0019]FIG. 6 is a flow chart of an exemplary method of practicing thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The following detailed description of the present inventionrefers to the accompanying drawings that illustrate exemplaryembodiments consistent with this invention. Other embodiments arepossible, and modifications may be made to the embodiments within thespirit and scope of the invention. Therefore, the following detaileddescription is not meant to limit the invention. Rather, the scope ofthe invention is defined by the appended claims.

[0021] It would be apparent to one of skill in the art that the presentinvention, as described below, may be implemented in many differentembodiments of hardware, software, firmware, and/or the entitiesillustrated in the figures. Any actual software code with specializedcontrolled hardware to implement the present invention is not limitingof the present invention. Thus, the operation and behavior of thepresent invention will be described with the understanding thatmodifications and variations of the embodiments are possible, given thelevel of detail presented herein.

[0022]FIG. 1 is a block diagram of the exemplary embodiment of thepresent invention. Included in FIG. 1 is a circuit 100 including aconverter 102 and a stabilizing circuit 106. The converter 102 isprovided to convert a single-ended input single V_(sig) received at aninput terminal 101, into a differential output signal V_(out). Thedifferential output signal V_(out) is then provided at respectivepositive and negative differential output terminals 104 and 105. Inorder to make the converter 102 symmetrical, an input terminal 103 isprovided for receiving a matching DC voltage signal V_(dc) as an input.That is, the signal V_(dc), provided from a DC voltage source, has avoltage level substantially equivalent to an average of the voltagelevel of signal V_(sig). The converter 102, as will be discussed ingreater detail below, receives the input signals V_(sig) and V_(dc) andconverts these signals into the differential output signal V_(out). Thesymmetry between the relationship of V_(sig) and V_(dc) minimizessubstrate noise since signals having roughly equivalent average signallevels are input at both sides of the converter 102. In an exemplaryembodiment, the degree to which the levels between V_(sig) and V_(dc)match is within about ±10%, although other tolerances may be selected.This symmetry provides a degree of balance to the converter and preventsthe occurrence of substrate and other noises appearing at the outputterminals 104 and 105.

[0023] As stated above, a popular technique for converting single-endedsignals to differential signals includes converting the receivedsingle-ended signal from the voltage domain to current domain and thenback to the voltage domain. Although this technique is not particularlywell suited for low voltage application, an inherent benefit iselimination of undesirable common-mode voltages. As known in the art,common-mode voltages may result from a number of different factors suchas improper circuit grounding or noise in the actual input source, etc.Converting single-ended signals to differential signals using thevoltage to current conversion technique removes these unwantedcommon-mode voltage components. In the present invention, however, thestabilizing circuit 106 is provided to remove unwanted common-modevoltage components, in place of the voltage to current conversiontechnique discussed above.

[0024] The stabilizing circuit 106 includes a sensor 107 and acomparator 108. The sensor 107 is configured to detect the common-modevoltage level of the differential output signal V_(out). In particular,the sensor 107 is coupled to the converter 102 via connection leads 109and receives the differential signal V_(out) as an input thereto. Thesensor 107 then provides a measurement of the common-mode voltage levelof the differential output signal V_(out) to the comparator 108. Thecomparator 108 also receives a reference common-mode voltage signalV_(cm) as an input from a reference voltage source (not shown). Thecomparator 108 compares the measured common-mode voltage level of thedifferential signal V_(out) with the reference common-mode voltagesignal V_(cm) and provides an adjusting signal V_(adj) at an outputterminal 110 of the comparator 108. The adjusting signal isrepresentative of the difference between the reference common-modevoltage signal V_(cm) and the common-mode voltage level of thedifferential output signal V_(out). The adjusting signal V_(adj) is thenprovided to the converter 102 along a feedback path 111 in order toadjust the common-mode voltage level of the differential output signalV_(out).

[0025] The output of the comparator 108 may be used in a number ofdifferent ways. The value of the adjusting signal V_(adj) output fromthe comparator 108 is equal to a gain (A) of the comparator 108multiplied by a difference between the voltage V_(cm) and thecommon-mode voltage level of the differential output signal V_(out). Inother words V_(adj)=A*(V_(cm)−V_(out)). The adjusting signal V_(adj) istherefore operative to adjust the common-mode voltage level of thedifferential signal V_(out), more specifically, a subsequent common-modevoltage level of V_(out).

[0026]FIG. 2 provides a more detailed view of the circuit 100, includingthe converter 102 and the stabilizing circuit 106. The converter 102includes an attenuator formed of two sets of impedance devices 206 a/206b and 208 a/208 b. The primary function of the impedance devices 206a/206 b and 208 a/206 b is to attenuate V_(sig) and convert V_(sig) to adifferential output signal. As shown in FIG. 2, the impedance devices208 a and 208 b are positioned adjacent to one another and connectedtogether in series. The impedance device 206 a is connected in serieswith the impedance device 208 a and the impedance device 206 b isconnected in series with the impedance device 208 b.

[0027] In a preferred embodiment of the present invention, the impedancedevices 206 a and 206 b have a substantially equal impedance value (Z₁),and the impedance devices 208 a and 208 b have a substantially equalimpedance value, (Z₂). Furthermore, the value of impedance devices 206 aand 206 b is chosen as a function of the desired degree of attenuationto be provided by the converter 102. A connection node between theimpedance devices 206 a and 208 a forms the output terminal 104 and aconnection node between the impedance devices 206 b and 208 b forms theoutput terminal 105. The signal input terminal 101 is formed by anunconnected side of the impedance device 206 a. Additionally, the signalinput terminal 103 is formed by an unconnected side of the impedancedevice 206 b.

[0028] As explained above, during operation of the circuit 100, thesingle-ended input signal V_(sig) is provided to the input terminal 101and the DC voltage signal V_(dc) is provided at the input terminal 103.Although the differential signal V_(out) may also be produced as aresult of applying only the input signal V_(sig) to the input terminal101, the input signal V_(dc) is provided at the input terminal 103 tomake the converter 102 symmetrical, as discussed above. When V_(sig) andV_(dc) are applied to respective input terminals 101 and 103 in a nearsimultaneous manner, the differential output signal V_(out) is producedat the output terminals 104 and 105.

[0029] The conversion technique used by the converter 102 provides asignificant improvement in linearity over the conventional approaches.For example, in video applications, a linearity of about 60 dB or betteris desirable to insure adequate processing of video signals. However,using the degenerate differential pair technique of the conventionalapproaches, the achieved linearity may be much less than 60 dB. In anexemplary embodiment of the present invention, however, linearities ofabout 80 dB are possible, over 20 dB better than the conventionalapproaches. Although the conversion technique of the present inventionprovides better linearity, it lacks the inherent common-mode rejectioncapability inherent in the conventional approaches. Therefore, thestabilizing circuit 106 is provided to suppress the common-mode voltageof the differential signal V_(out).

[0030] The common-mode sensor 107 of the stabilizing circuit 106includes a third set of impedance devices 210 a and 210 b, alsoconnected in series. In a preferred embodiment of the present invention,the impedance devices may be resistors or any other suitable devices.Also, the impedance devices 210 a and 210 b have a substantially equalimpedance value Z_(sense). Further, in most applications, the impedancevalue of the impedance devices 210 a and 210 b is higher than theimpedance value of the devices 208 a and 208 b. One end of the device210 a is coupled to the output terminal 104, and one end of theimpedance device 210 b is coupled to the output terminal 105. Connectedin this manner, the impedance devices 210 a and 210 b are arranged tomeasure the common-mode voltage of the differential output signalV_(out).

[0031] Also as shown in FIG. 2, a connection node 213 is formed betweenthe impedance devices 210 a and 210 b and is coupled to an invertinginput terminal 216 of the comparator 108. The comparator 108 may be forexample, an operational amplifier. The reference common-mode voltageV_(cm) is provided as an input to a non-inverting input terminal 214.The reference voltage V_(cm) may be provided by any conventional voltagegeneration means. Finally, the output terminal 110 is connected via thefeedback path 111, to a node 218 between the impedance devices 208 a and208 b, thus providing a negative feedback mechanism. The comparator 108receives the detected common-mode voltage from the sensor 107, comparesit with the reference common-mode voltage V_(cm), and adjusts thedetected common-mode voltage to match V_(cm). That is, the comparator108 provides a push pull arrangement which suppresses the common-modevoltage of the differential output signal V_(out), if its common-modevoltage is greater than V_(cm), and boosts the common-mode voltage ifthe common mode voltage is less than V_(cm).

[0032] Since most circuits that receive outputs from single-ended todifferential converters, such as track and hold circuits and ADCs, aresensitive to common-mode jump, an efficient technique is desirable forcontrolling the common-mode voltages associated with the differentialsignal V_(out). In the embodiment shown in FIG. 2, the adjusting signal,provided at the output terminal 110, is injected into the converter atthe node 218 in order to adjust the common-mode output of thedifferential output signal V_(out). The common-mode voltage associatedwith V_(out) can be adjusted up or down in accordance with the techniquediscussed above.

[0033] If the voltage values of V_(sig) and V_(dc) are suitable to drivethe entire circuit 100, they may be applied directly to input terminals101 and 103, as shown in FIG. 2. If, on the other hand, V_(sig) andV_(dc) are not suitable for driving the entire circuit 100 then voltagebuffers may be required at the input terminals 101 and 103 as shown inFIG. 3, to provide increased driving capability.

[0034]FIG. 3 is an illustration of the circuit arrangement of FIG. 2modified to include voltage buffers 300 and 304. The voltage buffers 300and 304 are respectively connected to the input terminals 101 and 103.The buffers 300 and 304 may be formed, for example, of operationalamplifiers. The voltage buffer 300 includes a non-inverting inputterminal 302 adapted to receive the input signal V_(sig) as an input andan inverting input terminal 303 coupled to a buffer output terminal 305.The output terminal 305 is connected to the input terminal 101 of theconverter 102. Similarly, the voltage buffer 304 includes anon-inverting input terminal 306 adapted to receive the input signalV_(dc) as an input and an inverting input terminal 308 connected to abuffer output terminal 309. The output terminal 309 is connected to theinput terminal 103 of the converter 102. In this arrangement, theattenuator formed by the impedance devices 206 a, 206 b, 208 a, and 208b loads the outputs of the buffers 300 and 304. The buffers thenattenuate V_(sig) in accordance with the following expression:$\frac{Z_{2}{Z_{sense}}}{Z_{1} + {Z_{2}{Z_{sense}}}}$

[0035] The differential output voltage V_(out) is the output of theconverter 102. As stated above however, if the signals V_(sig) andV_(dc) are adequate to drive at the circuit 100, then the buffers 300and 304 will not be needed.

[0036] The suitability of the converter 102, shown in FIGS. 2 and 3, foroperation in a low voltage environment depends on the implementation ofthe amplifiers used in the comparator 108 and the voltage buffers 300and 304. FIG. 4 shows an exemplary well known op-amp implementation thatis suitable for this application. The op-amp has respective invertingand non-inverting input terminals labeled (−in) and (+ip) and an outputterminal labeled (out).

[0037] Each circuit shown in FIGS. 2 and 3 is substantially internallysymmetrical except that each is driven in a single-ended manner.Symmetry is beneficial for rejection of power supply and substratenoise, with the symmetry being defined as the ability to swap V_(sig)and V_(dc), and the use of virtually identical circuit components oneach side of the converter 102 as shown, for example, in FIGS. 2 and 3.In the case of FIG. 2 and FIG. 3, although the converter 102 isconfigured as a single-ended to differential converter, it can also bedriven differentially by applying a differential signal to the voltagebuffers 300 and 304.

[0038] An exemplary embodiment of the present invention can also beconfigured to accommodate programmability. For example, FIG. 5 shows aconventional programmable gain attenuator 500, which can be directlyconnected to the output terminals 104 and 105 of the single-ended todifferential converter to permit a user to program specific attenuationvalues related to the output signal V_(out).

[0039]FIG. 6 illustrates a flowchart of an exemplary method ofpracticing the present invention. As shown in FIG. 6 and explained withreference to FIG. 2, the conversion process begins by the circuit 100receiving a single-ended signal at the input terminal 101 and a DCsignal at the input terminal 103 as described in block 600 of FIG. 6.Next, the single-ended signal is attenuated and a differential signal isproduced at the output terminals 104 and 105 as described in block 602.

[0040] The sensor 107 measures the common-mode voltage level of thedifferential signal and provides the detected common-mode voltage levelto the inverting input terminal 216 of the comparator 108 as describedin blocks 604 and 606 respectively. Next, the reference signal V_(cm) isprovided at the non-inverting input terminal the comparator 108, and iscompared with the detected common-mode voltage level of the differentialsignal as described in blocks 608 and 610 respectively. Finally, anadjusting signal is provided at the output terminal 110 of thecomparator 108 along the path 111. The adjusting signal is injected intothe converter 102 in order to adjust the subsequent common-mode voltagelevel of the differential signal based upon the adjusting signal asrespectively described in blocks 612, 614, and 616.

CONCLUSION

[0041] Using the technique of the present invention, a single-endedsignal can be converted into a differential signal in a low voltageenvironment having significantly improved linearity characteristics whencompared to conventional techniques. Also, the technique of the presentinvention can be used to adjust and suppress unwanted common-modevoltages in a stable manner. Finally the functions of single ended todifferential conversion, attenuation, and common-mode voltage adjustmentcan be combined and provided on a single IC to provide hardware savingson associated circuit boards.

[0042] The foregoing description of the preferred embodiments provide anillustration and description, but is not intended to be exhaustive or tolimit the invention to the precise form disclosed. Modification andvariations are possible, consistent with the above teachings or may beacquired from practice of the invention. Thus, it is noted that thescope of the invention is defined by the claims and their equivalents.

What is claimed is:
 1. A method to convert a single-ended signal to adifferential signal, comprising: attenuating a single-ended signalreceived within an attenuator and producing a differential signal atdifferential output ports formed within the attenuator, the differentialsignal being based upon the attenuated single-ended signal and areceived DC signal; detecting a common-mode voltage level of thedifferential signal and providing the detected common-mode voltage levelto an inverting input port of an amplifier; providing a referencecommon-mode voltage signal to a non-inverting input port of theamplifier; comparing the detected common-mode voltage level with avoltage level of the reference signal and providing an adjusting signalat an amplifier output port based upon the comparison; and adjustingcommon-mode voltage levels of subsequently produced differential signalsbased upon the adjusting signal.
 2. The method of claim 1, wherein anadjusted common-mode voltage level is substantially equal to the voltagelevel of the reference signal.
 3. An apparatus for converting asingle-ended signal to a differential signal, comprising: means forattenuating a single-ended signal received within an attenuator andproducing a differential signal at differential output ports formedwithin the attenuator, the differential signal being based upon theattenuated single-ended signal and a received DC signal; means fordetecting a common-mode voltage level of the differential signal andproviding the detected common-mode voltage level to an inverting inputport of an amplifier; means for providing a reference common-modevoltage signal to a non-inverting input port of the amplifier; means forcomparing the detected common-mode voltage level with a voltage level ofthe reference signal and providing an adjusting signal at an amplifieroutput port based upon the comparison; and means for adjustingcommon-mode voltage levels of subsequently produced differential signalsbased upon the adjusting signal.
 4. The apparatus of claim 3, wherein anadjusted common-mode voltage level is substantially equal to the voltagelevel of the reference signal.